Writing a Java Program with Native Methods

�� 忡�� Java�� ۼ�� α׷�� native �ڵ带 ��ս�Ű�� Ѵ�. ��, ��Hello World�� α׷�� õȴ�.

Backgrounds

Java ��α׷�� native �޼ҵ带 �ۼ��ϴ� �� .

step 1 : Write the Java code

Java Ŭ�� ۼ��Ѵ�. �ٸ� ��̶�� Ŭ�� ߿� �޼ҵ�� native�� ȴٴ� ��̴�.

step 2 : Compile the Java code

�Ϲ� �� javac�� ̿��ؼ� ��Ѵ�.

step 3 : Create the .h file

javah�� ̿��ؼ� ?jni �ɼ�� ָ� �� ִ�.

step 4 : Write the Native Method Implementation

native ��α׷�� ۼ��Ѵ�.

step 5 : Create a Shared Library

native ��α׷��  ��̺귯�� 鵵�� Ѵ�.

step 6 : Run the Program

java(java interpreter)�� ̿��ؼ� java ��α׷�� Ų��.

Step 1 : Write the Java code

�� Java �ڵ�� .

class HelloWorld {
    public native void displayHelloWorld();
    static {
        System.loadLibrary("hello");
    }

    public static void main(String[] args) {
        new HelloWorld().displayHelloWorld();
    }
}

Declare a Native Method

native Ű��带 ��ϰ� �Ǹ�, �� ޼ҵ忡 �� native �ڵ�� Լ�� ְ� �ȴ�. �̰�� Ŭ�� ο�� ϰ� �ȴ�. �� public ��̰�, ��ϰ�� μ�� Ǿ� �� ʴ�. �̰�� ϴ� �� ڿ�� ȴ�.

Load the Library

�� native �ڵ带 ��̺귯�� ؾ� �Ѵ�. �� ̺귯�� ̸�� System.LoadLibrary(library name)�� library name�� ؾ� �Ѵ�. �� hello�� Ǿ� �ִµ�, win32�� hello.dll��, Solaris �� libhello.so�� ȴ�. static�� Ǿ �� ʱ�ȭ�ϰ� �Ǵ� �Ϳ� �� ʿ䰡 �ִ�.

Write the Main Method

�Ϲ� �޼ҵ带 ��Ű�� Ͱ� �� ޼ҵ带 ȣ��ϸ� �ȴ�.

Step 2 : Compile the Java code

��ϴ� �� Ϲ�� ϴ� �� ϴ�.
��,

javac HelloWorld.java

�� ϸ� �ȴ�.

Step 3: Create the .h file

javah�� ϸ� �� ִ�. -jni �ɼ�� ֵ�� Ѵ�. -d �ɼ�� ϸ� �ٸ� ��丮�� ġ��ų �� ִ�.

The Function Definition

�� HelloWorld Ŭ��κ�� ̴�.

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloWorld */
#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: HelloWorld
* Method: displayHelloWorld
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld
(JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

�� 뿡�� native �ڵ忡�� ۼ��ؾ� �� Լ�� Ÿ�� Ǿ� �ִ�. ��Ÿ�� Ǵ� �� ׸�� .

JNIEnv �Ķ��͸� ��ؼ� native �ڵ尡 Java ��α׷�� Ѱ�� ڸ� �� ְ�, �ٽ� Java ��α׷�� ϰ�� Ѱ�� ְ� �ȴ�. jobject�� ü�� ǹ��Ѵ�. Java ��α׷�� this�� Ǵ� �Ͱ� ��ٰ� �� ִ�.

Step 4 : Write the Native Method Implementation

�� ڵ带 C ��ν� �ۼ�� . �� ڵ�� .

#include <jni.h>
#include "HelloWorld.h"
#include <stdio.h>
JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj)
{
printf("Hello world!\n");
return;
}

�� Եȴ�.

jni.h : Java ��α׷�� native ��α׷� �� interaction�� ؼ� �ʿ�� ִ� ��̴�.
HelloWorld.h : javah�� ̿��ؼ� �� ̴�. �� Լ�� Ÿ�� ǵǾ� �ִ�.
stdio.h : printf() �Լ�� ϱ� ��ؼ� ��Ե� ��̴�.

Step 5 : Create a Shared Library

Java ��α׷�� ڵ带 �ۼ�� ߴ�.

System.loadLibrary(��hello��);

�׷��Ƿ�, �� hello�� ̺귯�� ־�� Ѵ�. �̰�� տ�� ۼ�� C�� ν� �� ִ�.

Solaris�� 쿡�� ִ�.

cc -G -I/usr/local/java/include -I/usr/local/java/include/solaris HelloWorldImp.c -o libhello.so

Win32�� 쿡�� ִ�.

cl -Ic:\java\include -Ic:\java\include\win32 -LD HelloWorldImp.c -Fehello.dll

Step 6 : Run the Program

Java Interpreter�� ̿��ؼ� �ۼ�� α׷�� Ų��. �� Ų�ٸ�,

java HelloWorld

�� ϸ� �ȴ�.

�� ޽�� ߻��Ѵٸ� �н� �� ߸�� ־�� ̴�. �н� �� ٽ� Ȯ��ϰ� ��ؼ� ��ϸ� �ȴ�.

java.lang.UnsatisfiedLinkError: no hello in shared library path
        at java.lang.Runtime.loadLibrary(Runtime.java)
        at java.lang.System.loadLibrary(System.java)
        at java.lang.Thread.init(Thread.java)